This invention pertains to dry, free-flowing water-soluble chitosan-carboxylic acid complexes and a method for preparing them.
Chitosan, a deacetylated chitin, is a cation-active poly-primary amine with diverse applications in fields such as dispersing agents, adhesives, pesticides, waste water treatment, food processing, and wound healing. Chemically, chitosan is predominantly poly-1,4-(2-amino-2-deoxyglucose). Normally it is prepared from chitin by alkaline hydrolysis and is sufficiently deacetylated to become soluble in 1-4% acetic or formic acid. The parent chitin isolate is derived, for example, from crustacean shells by decalcification and deproteinization with acid and alkali, as appropriate.
Rigby patents, U.S. Pat. Nos. 2,040,879 and 2,040,880 (1936), are the classic references to the preparation of chitosan and list no fewer than 70 aqueous salt solutions thereof. A more recent reference describes chitosan salt solutions prepared similarly by dissolving chitosan in two molar aqueous solutions of carboxylic acids (Hayes et al. 1978; Mass. Inst. Tech., Office of Sea Grant, U.S. Department of Commerce, report MITSG 78-7, p. 103).
The art teaches the preparation by precipitation of dry water-soluble chitosan salts of inorganic acids. For example, Cupery in U.S. Pat. No. 2,201,762 teaches the precipitation of the sulfamic acid salt from an aqueous solution with methanol. Similarly, Sugano et al. in Am. J. Clinical Nutrition, 33, 787 (1980) teach the precipitation of the hydrochloride from an aqueous solution by the addition of acetone.
The art does not teach the preparation of dry, free-flowing, water-soluble normal and iso-monocarboxylic acid salts of chitosan. Cupery supra states the problem in using aqueous solutions as follows:
"attempts to prepare . . . the solid acetic acid derivative of . . . deacetylated chitin were not successful. A gel-like mass formed upon addition of the reaction mixture (a solution of about 4% chitosan in 4% aqueous acetic acid) to methanol." PA0 (A) combining to form a mixture; PA0 (B) maintaining said mixture until said chitosan is rendered water-soluble; and PA0 (C) recovering and drying water-soluble carboxylic acid complex of chitosan at a temperature below the denaturation temperature. PA0 (A) Combining to form a mixture: PA0 (B) maintaining said mixture until said chitosan is rendered water soluble; and PA0 (C) recovering and drying said salt at a temperature below the denaturation temperature. PA0 (1) Dispersing particulate chitosan in a selected liquid or mixture of liquids; PA0 (2) Adding to the dispersion a selected carboxylic acid, neat or in mixture with a liquid of (1); PA0 (3) Allowing the carboxylic acid to act on said chitosan in dispersion to form a chitosan-carboxylic acid complex; PA0 (4) Recovering and drying said salt at a temperature below the denaturation temperature.
As set out in Example 8, the technique of Sugano et al. which precipitated filterable chitosan hydrochloride by addition of acetone to its aqueous solution, failed when applied to organic acid salts of chitosan such as the salts of formic, glycolic, and lactic acid. The propionic acid salt was filterable but the product, on drying at room temperature, was water-insoluble, horny, and nonfree-flowing.
For pharmaceutical and nutritional purposes, for example, carboxylic acid salts are often preferred over inorganic acid salts such as the chloride.